Recently, attention has been increasingly drawn to a biomass refinery technique for systematically producing various chemicals using biomass as a raw material in place of fossil sources, such as petroleum. Particularly, levulinic acid and its esters, as chemicals producible from cellulose which is an inedible biomass source present in the greatest amount on the earth, are raw materials for basic chemicals including butene, for general-purpose resin sources including adipic acid, and for functional chemicals including agrochemicals, and their high potential as essential materials are attracting attention.
As methods for producing levulinic acid there are known methods for producing it by using sugar, such as glucose, or carbohydrate, such as starch or cellulose, as a raw material and heating the raw material in water into which hydrochloric acid, hydrobromic acid or sulfuric acid is added (see Non-Patent Literatures 1 and 2 and Patent Literatures 1 and 2). In either case, acid is required in a few or more equivalents per mole of sugar in the raw material and the product is also an organic acid, by reason of which how to prevent corrosion of the apparatus has become a major issue.
For example, Patent Literature 2 describes a method for continuously producing levulinic acid using cellulose as a raw material and sulfuric acid, in which case in order to produce levulinic acid at a yield of 70% or more, three or more equivalents of sulfuric acid is required per mole of glucose constituting cellulose. In the case of an industrial production process, how to prevent corrosion of the apparatus and the disposal of post-reaction acid become issues.
There is known as a solution to the above issues a method in which an alcohol is used as a reaction solvent to reduce the amount of acid used and synthesize a levulinic acid ester in a single stage from sugar, such as glucose, or carbohydrate, such as starch or cellulose. In this case, because the amount of acid used is required less than before and the product is an ester compound, the problems of corrosion of the apparatus and disposal of post-reaction acid are significantly reduced.
For example, according to a method in Patent Literature 3, a method is disclosed for synthesizing a levulinic acid ester from a cellulose-containing raw material, such as wood powder, using a catalyst quantity of organic acid with 10 or more carbon atoms in an alcohol solvent. In an example thereof, a method is disclosed in which about 20% by mole naphthalenesulfonic acid is used as a catalyst relative to cellulose in raw material wood powder and they are heated in an alcohol at 200° C. to synthesize a levulinic acid ester at a yield of 89 to 97%.
Furthermore, according to Non-Patent Literature 3, a reaction is disclosed in which methyl levulinate is synthesized from cellulose using a catalyst quantity of aluminum sulfate in methanol. In an example thereof, a method is disclosed in which about 16% by mole aluminum sulfate is used as a catalyst relative to cellulose as a raw material and they are heated in methanol at 180° C. to synthesize methyl levulinate at a yield of 44%.
According to methods described in Patent Literature 4 and an example of Non-Patent Literature 4, a reaction is disclosed in which methyl levulinate is synthesized from cellulose or a cellulose-containing raw material, such as wood powder, using a catalyst quantity of catalytic system consisting of a combination of a trifluoromethylsulfate of a Group XIII element and an organic sulfonic acid compound, at a higher yield than using each catalyst alone. For example, a method is disclosed therein in which a 0.8% by mole trifluoromethylsulfate of a Group XIII element and a 4% by mole aromatic sulfonic acid are used as catalysts relative to the raw material and they are heated in methanol at 180° C. to synthesize methyl levulinate at a yield of 65 to 75%. This method has an advantage in that the amount of acid catalyst used is further reduced, but practically has a cost problem because trifluoromethylsulfates are expensive.